When an electronic circuit is struck by lightning, and when it is connected to or disconnected from an inductive load or a large load, generally, a very high transient overvoltage is generated, and such a transient overvoltage is called a surge voltage, which is interference to the electronic circuit. In an existing surge protection design in the power supply industry, after the mains is connected, a surge protection circuit is designed at a high voltage side of a transformer so as to prevent a surge voltage from damaging a power supply.
Generally, a power supply includes a transformer, where a high voltage side of the transformer is an input end of the power supply, and a low voltage side of the transformer is an output end of the power supply.
As a result, a common power supply withstands a surge voltage by using a surge protection circuit at the high voltage side of the transformer, while the low voltage side is not protected by a surge protection circuit. When a surge voltage is generated on a terminal device, the terminal device discharges the surge voltage to a ground wire end at the low voltage side of the transformer through a protection component of the terminal device, so as to suppress the surge voltage on the terminal device by using an isolation component such as the transformer of the power supply. In this way, when energy of the surge voltage discharged at a terminal device side is lower than a suppression capability of the isolation component of the power supply, the power supply can withstand the surge voltage. However, when the energy of the surge voltage discharged at the terminal device side is higher than the suppression capability of the isolation component of the power supply, the isolation component of the power supply cannot withstand the surge voltage of such high energy and is broken down. As a result, the power supply is damaged.